New York led the charge in the 1880s to trade the hangman's noose for the electric chair, based on a legislative commission's finding that electrocution was "the most humane and practical method known to modern science," and other states followed its lead. In 1921, Nevada adopted a new method -- lethal gas -- after concluding that it was "the most humane manner known to modern science." And other states followed suit.

Though some states kept the firing squad and hanging, electrocution remained the predominant method of state execution for much of the 20th century. After the court upheld capital punishment in 1976, lethal injection was eventually adopted in most states, yet again, in an effort to find a more "humane" way to carry out death sentences.

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Because some risk of pain is inherent in any method of execution, the court has held time and again "that the Constitution does not require the avoidance of all risk of pain," Alito wrote.

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"Holding that the Eighth Amendment demands the elimination of essentially all risk of pain would effectively outlaw the death penalty altogether."

On that point alone, doctors and death penalty opponents might agree with Alito. As many opponents of capital punishment see it, the term "humane execution" is an oxymoron, something that cannot be achieved.

"Every time someone introduces a new method of execution, they make the same argument: 'Trust me. It will be quick. It will be painless. It is the most humane alternative,' " said Robert Dunham, director of the Death Penalty Information Center.

"They have a political or commercial interest in the outcome. And they haven't done any real medical or scientific research to back up their claims. And in every instance, sooner or later, something they didn't anticipate goes wrong."

Nowhere in the Constitution does it say that capital punishment need be "humane." As the Connecticut Supreme Court's recent ruling to commute the sentences of death row inmates to life without parole demonstrates, a substantial portion of recent death penalty challenges have been based upon the Eighth Amendment's prohibition of "cruel and unusual punishments."

Death penalty opponents caution against conflating the two terms in the capital punishment debate. Invoking the word "humane" runs the risk of appearing soft on criminals or insensitive to victims' suffering, distracting from the real issue, the say. For that reason, many in the medical community want to remove the term from debate altogether, arguing that a death sentence can never be humane.

A conflict of interest

It may be medically possible to mitigate pain, but it's impossible to guarantee that any form of execution will be quick and painless, according to doctors who spoke with CNN on the issue.

Others refused to comment on the topic, given the profession's stance against capital punishment as anathema to its life-preserving mission.

"From a technical point of view, yes, it's possible to make an execution quick and as painless as possible, if that's what you're trained to do," said medical ethicist Dr. Daniel P. Sulmasy, associate director of the University of Chicago's MacLean Center for Clinical Medical Ethics.

From an ethical perspective, however, "it is always immoral and inhumane to participate in an execution, even if it's done painlessly," he said. "Our job is to be life-affirming, and participating in an execution is inconsistent with that mission because it ends up making us agents of the state in killing people."

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Lethal injection in particular is problematic because it relies on medical technology and knowledge -- syringes, IV poles lines, finding usable veins -- even though it's not a medical procedure, said Dr. Marc Stern, who served as medical director for Washington State's Department of Corrections from 2006 to 2008. Each state comes up with its own protocol without a set of standardized guidelines developed by medical professionals.

"Unless it's an open medical procedure subject to all the checks and balances in medical science, you're going to have botched executions," Stern said.

What's left is an "impersonation" of medicine that would be considered criminal in any other setting, said Dr. Joel B. Zivot, assistant professor of anesthesiology and surgery at the Emory University School of Medicine in Atlanta.

Unlike in physician-assisted suicide, which is legal in four U.S. states and has the backing of a controlling legal authority, there is no uniform policy of medical oversight and no guarantee that licensed medical professionals will be involved in carrying out death sentences, Zivot said. Most critically, perhaps, is the absence of free will.

Each method comes with its own set of risks, Zivot said. But lethal injection does the best job of creating the "illusion" of a peaceful execution by rendering the inmate unconscious with a sedative before delivering the drugs that paralyze and stop the heart.

"Since we cannot ask the person now dead if the method was cruel or painless, the only way to tell is based on the way it appears to observers," Zivot said.

"If we took away the sedative, I expect we would see the real face of execution, the consequence of paralysis and suffocation, and I imagine we would observe that experience to be cruel."

Is there a doctor in the big house?

The Society of Correctional Physicians, which represents medical professionals working in prisons, affirms the American Medical Association's opinion that physicians should not be participants in legally authorized executions.

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Although physicians in correctional medicine serve the facilities in which they work, "participation in any aspect of implementation of the death penalty does not increase the safety or security of the institution," the group's position statement says.

"Physicians, while possessing special expertise in the use of medications and knowledge of the human body, should not utilize this skill and knowledge in assisting in the killing of human beings," the statement says. "Such participation or supervision at any level is not appropriate to our professional role."

Participation can include administering drugs as part of the execution protocol, monitoring vital signs or, in the case of lethal injection, starting intravenous lines as a port for a lethal injection device. It does not include certifying death after the condemned has been declared dead.

However, the position statement is a "guideline" that physicians can point to if they feel pressured by their employers to participate in executions. It's not a mandate; the decision to participate ultimately rests with the individual.

Why some physicians take part

A 2006 article in the New England Journal of Medicine addressed why some physicians participate in executions. Four physicians and a nurse, speaking under the condition of anonymity, said they saw it as an extension of their obligation to patients to ensure that the process went as smoothly as possible.

"It was my responsibility to make sure that everything be done in a way that was professional and respectful to the inmate as a human being," a nurse said. "If this is to be done correctly, if it is to be done at all, then I am the person to do it."

Otherwise, thanks to laws limiting public disclosure of the people and processes involved, it's hard to know for sure how often licensed medical staff participates in executions, said Stern.

It leaves medical professionals deciding between staying true to their ethical obligations and keeping their job, as Stern knows firsthand.

He was responsible for all health care services in the state's correctional facilities in 2008, when he learned that drugs for a scheduled execution were acquired through the penitentiary without his permission. He asked the department to return the chemicals to the pharmacy to avoid implicating him or his staff in the execution.

"My purpose was not to stop the execution but to remove myself and my staff from involvement," he said. "The only way to honor my ethical obligation was to resign."

And still, executions continue in the United States, often with the participation of licensed medical staff, as the death of Oklahoma inmate Clayton Lockett revealed.

'Procedural disaster'

Lockett's execution, the state's first using the sedative midazolam in its three-drug protocol, inspired the Eighth Amendment claim the Supreme Court decided in June. It turned out to be a "procedural disaster," in the words of the Tenth Circuit Court of Appeals, prompting the state to revise its safety protocol.

A review of the execution determined that the team, which included a doctor and paramedic, failed to properly establish intravenous access to Lockett's cardiovascular system, causing the IV fluid to leak into tissue rather then enter his bloodstream.

After the team declared Lockett unconscious from the midazolam, it proceeded with the injection of the vecuronium bromide and potassium chloride. Shortly after the injection of the potassium chloride, Lockett began to move and speak. Witnesses reported Lockett cursing and moaning, "the drugs aren't working."

The team stopped the potassium chloride injection, and Lockett was pronounced dead about 43 minutes after the midazolam was first injected.

The execution caused Oklahoma to implement new safety precautions as part of its lethal injection protocol. When Oklahoma executed Lockett, its protocol called for the administration of 100 milligrams of midazolam, compared with the 500 milligrams that are currently required. It also prompted Oklahoma to implement new safety precautions as part of its lethal injection protocol.

The outcome prompted three Oklahoma inmates to challenge the state's use of midazolam in a case that went all the way to the Supreme Court. In a 5-4 ruling, the justices found that the prisoners failed to show that using midazolam creates a risk of severe pain or that the risk is substantially greater compared to known and available alternatives.

Midazolam, a benzodiazepine similar to Valium and Xanax, is the latest substitute to be used in place of fast-acting barbiturates sodium thiopental and pentobarbital, which are becoming increasingly scarce as drug companies refuse to provide them for use in capital punishment.

Oklahoma even has a backup, just in case. While the Supreme Court case was pending, Gov. Mary Fallin signed a bill that would allow the state to perform executions with nitrogen gas if lethal injection is ruled unconstitutional or becomes unavailable.

While the medical community has voiced concerns about the method, at least one group thinks the Sooner State might be onto something.

Philip Nitschke, director of the right-to-die group Exit International, said the increasing difficulty in obtaining pentobarbital has prompted him to consider gas as an alternative. He is working on a self-activated "destiny machine" that will deliver a combination of nitrogen and carbon monoxide through nasal prongs to instigate hypoxia.

The difference between his machine and Oklahoma's method? Free will, he said.

"I suspect there will still be horror stories," he said. "You still have to approach someone who doesn't want to die."

There are no professional executioners

Even if you take out the emotional or philosophical questions about capital punishment, the potential for operator error remains, said Fordham University School of Law professor Deborah W. Denno.

Just because physicians, nurses and licensed medical staff participate directly in lethal injection by finding intravenous access or administering the drugs, there's still a chance something can go wrong, especially outside a clinical setting. If they are not involved at all, the potential for error increases, said Denno, one the nation's foremost death penalty experts.

The same goes for electrocutions, hangings, firing squads, even the guillotine, she said. Unless there's an electrical engineer, a hangman, a marksman or a trained guillotine operator running the show, the margin of error increases, Denno said.

"This has always been the problem throughout history: the people performing the executions," she said. "Why? We don't train people to be executioners." The closest thing America has to trained executioners are shooters or marksmen, theoretically making a firing squad the best option for successful outcomes, she said.

But who wants to witness a firing squad? "People are horrified by firing squads because they look so bad, or they get associated with authoritarian regimes," she said. "Utah is the only (state) that has it, and they've been mocked endlessly each time."

Guillotines are subject to the same perception problems. "When there's an ISIS beheading, no one ever says 'at least they didn't suffer,' " said Austin Sarat, professor of jurisprudence and political science and associate dean of faculty at Amherst College.

Sarat analyzed 8,776 executions performed in the United States between 1900 and 2010 for his book, "Gruesome Spectacles: Botched Executions and America's Death Penalty." He came up with a botch rate of 3.15%, or 276 executions.

The method with the highest rate of error based on his research? Lethal injection, he said. The lowest: the electric chair.

"The legitimacy of the death penalty is sustained by the illusive search for painless executions," he said. "We cannot be relied on to deliver executions which impose no more than pain than is necessary, and we certainly can't be relied on to impose executions that are compatible with contemporary standards of decency."